Transceivers are electronic devices that include a transmitter and a receiver and often use a common antenna to transmit and receive radio waves. The transmitter and receiver of a transceiver may include common circuitry or be separate devices, e.g., within a common housing or in separate housings. When an antenna presents a single interface to both a transmitter and a receiver, a transmit/receive switching function can be used to switch between transmit and receive modes of operation.
FIG. 1 is a schematic diagram of a conventional transmit/receive circuit 100 for an RF port 130. The RF port 130 electrically connects a PA 110 and a low noise amplifier (LNA) 140 to an antenna. The circuit 100 also includes a switch 120 that selectively allows signals output by the PA 110 to be applied to the antenna and a switch 150 that selectively allows the LNA 140 to receive signals from the antenna. For example, during transmit mode, the switch 120 may be closed to allow the PA output signals to reach the antenna by way of the RF port 130. The switch 150 may be open during transmit mode to provide a high impedance into the LNA (e.g., as seen from the RF port 130) and prevent PA output signals from reaching the LNA 140. During receive mode, the switch 150 may be closed to allow antenna signals to reach the LNA 140 and the switch 120 may be open to provide a high impedance into the PA 110 (e.g., as seen from the RF port 130) and prevent antenna signals from reaching the PA 110.
In this direct series configuration, the switch 120 provides a loss mechanism for the PA output signals. For example, the switch 120 and the antenna form a voltage divider network such that some of the voltage output by the PA 110 is loss at the switch 120. In addition, voltage swings on the PA side of the switch 120 should be limited in this configuration due to supply voltage and reliability considerations. For example, high voltage swings at the switch 120 can reduce the reliability of the switch 120. Also, the parasitic capacitance of the switch 120, especially if precautions are needed for ESD protection, can load the RF port 130 and can degrade the noise figure in receive mode.